Closed cycle waste combustion

ABSTRACT

An apparatus for treating organic waste material characterized by high ash content is disclosed. The apparatus includes a slagging combustor for burning the organic waste material to produce a slag of molten inorganic ash and exhaust gases, a cooler for receiving the exhaust gases from the combustor and cooling the exhaust gases, a condenser for receiving cooled exhaust gases from the cooler and drying the cooled exhaust gases, an exhaust gas recirculation conduit for receiving a first portion of cooled and dried exhaust gases from the condenser, and a source of concentrated oxygen gas in fluid communication with the exhaust gas recirculation conduit for adding concentrated oxygen gas to the first portion of cooled and dried exhaust gases to create a gas mixture that is added to the combustor through the exhaust gas recirculation conduit, wherein the source of concentrated oxygen gas includes a valve responsive to an oxygen sensor in the exhaust gas recirculation conduit for regulating the flow of concentrated oxygen gas into the exhaust gas recirculation conduit.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part application of U.S.patent application Ser. No. 09/489,081 filed Jan. 21, 2000 which is adivisional application of U.S. patent application Ser. No. 09/055,502,filed Apr. 6, 1998, now U.S. Pat. No. 6,029,588.

BACKGROUND OF THE INVENTION

[0002] This invention relates to the combustion of organic wastematerial, and particularly to a closed cycle combustion of wastematerial using concentrated oxygen.

[0003] Waste materials such as municipal solid waste, waste watertreatment sludge, and paper mill sludge, are often treated byincineration. Such waste material contains organic combustible matterand inorganic metal oxides. The organic combustible matter typicallyprovides sufficient thermal energy during combustion to maintain highcombustion chamber temperatures without the need for supplemental fuel.The inorganic portion of the waste material is characterized by thepresence of some silica (SiO₂) and other glass forming metal oxides. Ifa slagging combustor such as a rotary kiln or cyclone furnace is usedfor combustion, the inorganic portion of the waste material can reach atemperature high enough to melt. The resulting molten material isdrained from the combustion chamber as slag.

[0004] Conventional incinerators designed to combust organic wastematerial use air as the oxidizer source. Since almost four-fifths of airis inert gases (primarily nitrogen), a major portion of the air providesno benefits to the combustion process. In fact, the inert gas causesseveral distinct disadvantages. A first disadvantage is that thecombustion flame temperature is lowered, thereby making it difficult tomaintain the necessary temperatures to melt the inorganic metal oxidesin the waste material. Secondly, the waste gases from the incinerationwill be contaminated with substantial amounts of nitrogen that resultsin a large volume of exhaust gases which require further treatmentbefore release into the atmosphere.

[0005] It has been proposed to reduce the undesirable effects ofnitrogen in the incineration of hazardous waste by introducingconcentrated oxygen into the combustion chamber along with recycledexhaust gases. See U.S. Pat. No. 5,309,850 issued May 10, 1994, toDowns, et al.

[0006] The present invention also uses concentrated oxygen in a closedcycle to treat non-hazardous waste and to convert the waste materialinto useful end products.

SUMMARY OF THE INVENTION

[0007] In accordance with the invention, the non-hazardous organic wastematerial is introduced into a slagging combustor where it is burned. Theburning produces exhaust gases and a slag of molten, inorganic ash whichis removed from the combustor. The exhaust gases are treated to remove amajor portion of particulate matter contained therein. A portion of thetreated exhaust gases is mixed with a source of concentrated oxygen in aproportion that results in mixed gases having an oxygen concentration ofat least 30% by volume. The mixed gases are introduced into thecombustor to support the burning of the waste material.

[0008] Preferably, the proportion of oxygen in the mixed gases is fromabout 40% to 50% by volume. The exhaust gases may be cooled and driedbefore mixing with the concentrated oxygen.

[0009] Further in accordance with the invention, a second portion of thetreated exhaust gases may be treated to remove the carbon dioxidetherefrom. The removed carbon dioxide is preferably converted into aliquid form.

[0010] Also in accordance with one embodiment of the invention, aportion of the heat from the exhaust gases is transferred to the mixedgases before the mixed gases are introduced into the combustor.

[0011] The invention further comprises apparatus for carrying out themethod.

[0012] The resulting products of the process of the invention areuseful. The liquefied carbon dioxide can be marketed and utilized as aproduct. The carbon dioxide thus produced would displace carbon dioxidethat is currently produced using natural gas or other natural resourcesthereby conserving on natural resources. The inorganic products in thewaste material are vitrified into a highly inert granular material whichmay be used as a construction material. Conventional waste materialincinerators generally produce ash that must be land filled. With theexception of a small amount of non-condensible gas at the exit of thecarbon dioxide recovery system, there are no emissions into the air andthe environmental impacts are insignificant as compared to conventionalincineration processes which have significant emissions.

[0013] The foregoing and other objects and advantages of the inventionwill appear in the detailed description which follows. In thedescription, reference is made to the accompanying drawings whichillustrate a various embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a schematic diagram of the apparatus for carrying outthe invention. FIG. 2 is a schematic diagram of another embodiment of anapparatus for carrying out the invention. FIG. 3 is a schematic diagramof yet another embodiment of an apparatus for carrying out theinvention. FIG. 4 is a schematic diagram of still another embodiment ofan apparatus for carrying out the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0015] Referring to FIG. 1, dry waste material (with moisture contentlow enough to support good combustion) is introduced through line 7 intomixer 9. With some waste materials, it may be necessary to add eitherfluxing agents, glass forming materials (such as SiO₂), or both tooptimize melting point and to assure good quality of glass slagproduced. The fluxing agent and/or glass forming material are introducedin line 8 in the mixer 9. The mixed material is introduced through line10 into combustion chamber 11.

[0016] The waste material may consist of paper mill sludge, municipalwaste water treatment sludge, municipal solid waste, or like materials.The waste material is characterized by a heating value lower thanconventional fuels and by an ash content that is higher thanconventional solid fuels such as coal. The heating value will typicallyrange, but is not limited to, values of 500 Btu/lb to 9,000 Btu/lb. Ashcontent will typically range from 5% to 65%. Combustion chamber 11 is arefractory lined chamber. The combustion chamber is designed to promotegood contact of the waste material and the gas source. The combustionchamber may be a water cooled combustion chamber, a cyclone furnace, ora rotary kiln. The average operating temperature of the combustionchamber will normally range from 2,500° F. and 3,500° F. The operatingtemperature inside the combustion chamber 11 will be hot enough to causethe inorganic ash in the waste material to melt into a fluid state. Themolten inorganic ash is drained through the bottom of the combustionchamber 11 by a line 12, where the slag is quenched. The spentcombustion exhaust gas exits the combustion chamber through a line 13 ata temperature of 2,500° F. to 3,500° F. and enters a mixing chamber 14.The hot exhaust gases mix with cool recycled gases that enter from aline 33. The flow of cool recycled gas is moderated to control the gastemperature exiting the mixer 14 through a line 15 to a temperature of750° F. to 1,400° F. In an alternate arrangement, the mixing chamber 14would be replaced with a steam boiler.

[0017] The exhaust gas from line 15 enters a gas-to-gas heat exchanger16 where heat is transferred from the exhaust gas to regenerated andrecycled combustion gas. The heat exchanger 16 is desirable but optionaldepending on the operating parameters of the system. The exhaust gasthen proceeds through a line 17 to a steam boiler or water heater 18 inwhich additional cooling of the exhaust gas will occur. Feedwater entersthe boiler 18 through a line 19 and steam exits through a line 20. Thecool combustion gas leaves the steam boiler 18 through a line 21 andenters a particulate filter 22 where fine particulate matter is capturedand removed from the system through a line 23. The particulate freeexhaust gases exit the filter through a line 24 and enter a water vaporcondenser 25. Cool circulating water enters via a line 26 and exits viaa line 27. A major portion of the water vapor condenses out of theexhaust gas steam and is drained through a line 28. The vapor condenser25 is preferably constructed from corrosion resistant materials. Thevapor condenser will also further remove particulate matter not capturedin the particulate filter 22.

[0018] After most of the water vapor has been removed, the exhaust gasexits through a line 29. At this point in the process most (75% to 95%by volume) of the process gas stream is carbon dioxide (CO₂) along withsmall amounts of nitrogen (N₂), oxygen (O₂), and water vapor (H₂O). Theprocess gas stream will also contain trace amounts of nitrogen dioxide(NO₂), sulfur dioxide (SO₂), volatile organic compounds (H_(x)C_(y)),hydrogen chloride (HCl), carbon monoxide (CO) and particulate matter.

[0019] A first portion of the gas stream is recirculated back into thecombustion loop through a line 31, with the remainder of the gas streamproceeding through a line 30 for further processing. The mass flow rateof carbon dioxide through line 30 is equal to the amount of carbondioxide formed during the combustion phase of the process under steadystate conditions. The first portion of the gas flow that is to berecirculated enters a fan 32 which provides the necessary head toovercome pressure losses as the gas flows through the closed loop. Thegas flow exits fan 32 and splits into lines 33 and lines 34. The gasflow in line 34 mixes with concentrated oxygen in a line 40 leading froma source 38. The concentration of oxygen in the line 40 will normallyrange from 90% to 95% oxygen by volume. Line 35 receives the mixed gasstream from lines 34 and 40. The mixed gas has now been regenerated andcontains sufficient oxygen concentration for combustion. Typical oxygenconcentrations in the regenerated gas stream can range from 30% to 80%oxygen by volume, with optimum concentrations of 40% to 55%. The desiredoxygen concentration in the regenerated gas stream is selected based onmaintaining optimum combustion temperatures and combustion efficiency inthe combustion chamber 11. The desired oxygen concentration may varywith waste fuel, combustion technology, and other operating factors. Theamount of oxygen in the mixed gas stream is sensed by an oxygen sensor57 and is controlled by a valve 58 in line 40.

[0020] The regenerated gas in line 35 enters the gas-to-gas heatexchanger 16 where it receives heat from the exhaust gas. A highertemperature in the regenerated gas will enhance combustion performance.The temperature of the regenerated gas will normally range from 400° F.to 1200° F. The heated regenerated gas enters a line 36 where itproceeds to the combustion chamber 11.

[0021] The concentrated oxygen is generated in an air separation unit38, which accepts air through line 37 and separates oxygen (O₂) fromnitrogen (N₂). The oxygen exits through line 40 while the nitrogen isvented back to the atmosphere through a line 39. The art of airseparation is well established. Air separation can be performed by anynumber of methods, such as vacuum pressure swing absorption, orcryogenic air separation. Either method can provide a suitable supply onconcentrated oxygen.

[0022] In special circumstances where the recovery of carbon dioxide isnot desired, a second portion of the exhaust gas from line 30 may bevented directly to the atmosphere or through a final filter (not shown)and then to the atmosphere.

[0023] If carbon dioxide is to be recovered, the excess gas in line 30proceeds to a gas clean up system 41. The presence of a number of tracegases may impact the product quality and marketability. The trace gaseswould include nitrogen dioxide (NO₂), sulfur dioxide (SO₂), hydrogenchloride (HCl), hydro carbon based gases (H_(x)C_(y)), and carbonmonoxide (CO). The presence and concentration of the various compoundswill be a function of the waste fuels consumed and the operatingparameters of the combustion system. In practice, system 41 wouldconsist of several steps, and would likely include, but is notnecessarily limited to: heat exchangers for modifying the gastemperature, gas heaters, catalyst beds (for reducing trace gases suchas N0 ₂, CO, H_(x)C_(y), into N₂, H₂O and CO₂), scrubbers (for directremoval of HCl and SO₂ with the use of reagents), dehumidifiers ordesiccant dryers (for removal of water vapor), and final filters (forremoval of any fine particulate matter). The sequence and selection ofthe various removal equipment is known in the art and will vary with theinitial concentrations of the trace gases and what end productspecifications are desired.

[0024] The cleaned gases exit system 41 into a line 42 and proceed to acompressor 43. The gas pressure at the inlet to the compressor is at orbelow 1.0 atmospheres (14.7 psia). To provide for proper conditions toallow the carbon dioxide to liquefy, the compressor 43 compresses thegas to pressures of 20 to 65 atmospheres. The compressed gas exitsthrough a line 46. The compressor is cooled with water from a line 44,and the heated water line leaves via a line 45.

[0025] The compressed gas enters a heat exchanger 48, where the gas iscooled indirectly with refrigerant furnished through a line 47. Therefrigerant temperature will typically range from 30° F. to minus 30° F.depending on initial gas compressor operating pressure and the desiredcarbon dioxide removal efficiency. A portion of the carbon dioxide istransformed from a gas to a liquid and drained out through a line 49.Nitrogen and oxygen, along with some carbon dioxide that was notliquefied in the first stage, exhaust through a line 50 and enter a heatexchanger 52. Refrigerant from a line 51, which would typically rangefrom 0° F. to minus 55° F., will further cool the exhaust gases andliquefy additional carbon dioxide. The additional carbon dioxide exitsthrough a line 53 and is combined with that in line 49 to a line 55. Thecarbon dioxide in line 55 would be handled as a conventional liquidcarbon dioxide product. Gas exiting via a line 54 is vented and willconsist primarily of nitrogen and oxygen along with a small percentageof carbon dioxide that was not liquefied.

[0026] The second stage of separation (heat exchanger 52) is optionaland its need is based on the desired CO₂ collection efficiency. If thesecond stage of separation is not utilized, line 50 would vent to theatmosphere.

[0027] Supplemental fuels such as natural gas, propane, petroleum oil,wood, and coal may be added to the combustion chamber 11 through a line60 to maintain the temperature necessary to melt the inorganic material.

[0028] In FIG. 2, there is shown another apparatus for carrying out theinvention. This apparatus differs from the apparatus of FIG. 1 in thatrather than mixing the concentrated oxygen from the source 38 with therecirculated gas in line 34 as in the apparatus of FIG. 1, the apparatusof FIG. 2 introduces oxygen directly into the combustion chamber 11through a line 59. The oxygen concentration of the gases entering thecombustion chamber 11 is maintained at the same levels discussed abovewith respect to the regenerated gas stream of FIG. 1 (30-80%) by way ofthe valve 58 which is responsive to the oxygen sensor 57.

[0029] In FIG. 3, there is shown yet another apparatus for carrying outthe invention. The apparatus of FIG. 3 differs from the apparatus ofFIG. 2 in that the oxygen sensor 57 is relocated from line 59 to line15. Therefore, the apparatus of FIG. 3 provides an alternative locationfor sensing oxygen in the apparatus by way of the oxygen sensor 57. Thevalve 58 in line 40 is responsive to the oxygen sensor 57 in order tomaintain the oxygen concentration of gas entering the chamber 11 at thelevels discussed above for FIG. 1 (30-80%).

[0030] In FIG. 4, there is shown still another apparatus for carryingout the invention. The apparatus of FIG. 4 differs from the apparatus ofFIG. 2 in that the oxygen sensor 57 has been removed from line 59, afirst flow sensor 60 has been installed in line 40, and a second flowsensor 61 has been installed in line 36. In the apparatus of FIG. 4, thefirst flow sensor 60 measures the fluid flow in line 40, and the secondflow sensor 61 measures the fluid flow in line 36. By measuring thefluid flows in lines 36 and 40, the volumetric percentage of oxygen canbe calculated (such as in a system controller), and the calculatedresult can be used to control the valve 58 in line 40. In this manner,the valve 58 is responsive to the calculated oxygen values from thefirst flow sensor 60 and the second flow sensor 61 such that the oxygenconcentration of gases entering the chamber 11 is maintained at the samelevels discussed above with respect to the regenerated gas stream ofFIG. 1 (30-80%).

I claim:
 1. An apparatus for treating organic waste materialcharacterized by high ash content and a heat value of about 500 to about9,000 Btus per pound, the apparatus comprising: a slagging combustor forburning the organic waste material to produce a slag of molten inorganicash and exhaust gases; a cooler in fluid communication with thecombustor, the cooler receiving the exhaust gases from the combustor andcooling the exhaust gases; a condenser in fluid communication with thecooler, the condenser receiving cooled exhaust gases from the cooler anddrying the cooled exhaust gases; a condenser gas output conduit in fluidcommunication with the condenser; an exhaust gas recirculation conduitin fluid communication with the condenser gas output conduit and thecombustor, the exhaust gas recirculation conduit receiving a firstportion of cooled and dried exhaust gases from the condenser gas outputconduit and recirculating the first portion of cooled and dried exhaustgases to the combustor; a source of concentrated oxygen gas in fluidcommunication with the combustor for adding concentrated oxygen gas tothe combustor; and an oxygen sensor for measuring the oxygen in theexhaust gases, wherein the source of concentrated oxygen gas includes avalve responsive to the oxygen sensor, the valve being suitable forregulating a flow of concentrated oxygen gas in response to the oxygensensor.
 2. The apparatus of claim 1 wherein the source of concentratedoxygen gas comprises: an air separator for separating concentratedoxygen gas from ambient air input.
 3. The apparatus of claim 1 furthercomprising: a particulate filter in fluid communication with the coolerand the condenser, the particulate filter receiving cooled exhaust gasesfrom the cooler and removing particulate matter from the cooled exhaustgases before the cooled exhaust gases enter the condenser.
 4. Theapparatus of claim 1 further comprising: a blower located in the exhaustgas recirculation conduit for increasing the pressure to induce flow ofthe first portion of cooled and dried exhaust gases.
 5. The apparatus ofclaim 1 further comprising: a gas heat exchanger having a first sectionin fluid communication with the combustor and the cooler and a secondsection in fluid communication with the exhaust gas recirculationconduit and the combustor, the gas heat exchanger transferring heat fromthe exhaust gases from the combustor to the gas mixture in the exhaustgas recirculation conduit.
 6. The apparatus of claim 5 furthercomprising: a gas mixer in fluid communication with the combustor andthe first section of the heat exchanger and in fluid communication withthe exhaust gas recirculation conduit, the gas mixer mixing the exhaustgases received from the combustor with an amount of the first portion ofcooled and dried exhaust gases received from the exhaust gasrecirculation conduit.
 7. The apparatus of claim 1 further comprising:an exhaust gas recovery conduit in fluid communication with thecondenser gas output conduit for receiving a second portion of cooledand dried exhaust gases from the condenser gas output conduit; and acarbon dioxide removal system in fluid communication with the exhaustgas recovery conduit, the carbon dioxide removal system receiving thesecond portion of cooled and dried exhaust gases from the exhaust gasrecovery conduit and recovering carbon dioxide from the second portionof cooled and dried exhaust gases.
 8. The apparatus of claim 7 whereinthe carbon dioxide removal system comprises: a compressor in fluidcommunication with the exhaust gas recovery conduit for compressing thesecond portion of cooled and dried exhaust gases received from theexhaust gas recovery conduit; and a recovery heat exchanger in fluidcommunication with the compressor, the recovery heat exchanger receivingthe compressed second portion of cooled and dried exhaust gases from thecompressor and recovering liquid carbon dioxide from the compressedsecond portion of cooled and dried exhaust gases.
 9. The apparatus ofclaim 8 wherein the carbon dioxide removal system further comprises: agas cleaner in fluid communication with the exhaust gas recovery conduitand the compressor, the gas cleaner receiving the second portion ofcooled and dried exhaust gases from the exhaust gas recovery conduit andremoving trace gases from the second portion of cooled and dried exhaustgases before the second portion of cooled and dried exhaust gases entersthe compressor.
 10. An apparatus for treating organic waste materialcharacterized by high ash content and a heat value of about 500 to about9,000 Btus per pound, the apparatus comprising: a slagging combustor forburning the organic waste material to produce a slag of molten inorganicash and exhaust gases; a cooler in fluid communication with thecombustor, the cooler receiving the exhaust gases from the combustor andcooling the exhaust gases; a condenser in fluid communication with thecooler, the condenser receiving cooled exhaust gases from the cooler anddrying the cooled exhaust gases; a condenser gas output conduit in fluidcommunication with the condenser; an exhaust gas recirculation conduitin fluid communication with the condenser gas output conduit and thecombustor, the exhaust gas recirculation conduit receiving a firstportion of cooled and dried exhaust gases from the condenser gas outputconduit and recirculating the first portion of cooled and dried exhaustgases to the combustor; a first flow sensor located in the exhaust gasrecirculation conduit; a source of concentrated oxygen gas in fluidcommunication by way of an oxygen conduit with the combustor for addingconcentrated oxygen gas to the combustor; and a second flow sensorlocated in the oxygen conduit, wherein the source of concentrated oxygengas includes a valve responsive to the first flow sensor and the secondflow sensor, the valve being suitable for regulating a flow ofconcentrated oxygen gas in response to the first flow sensor and thesecond flow sensor.
 11. The apparatus of claim 10 wherein: the firstflow sensor and the second flow sensor measure fluid flow and thevolumetric percentage of oxygen is calculated in order to control thevalve.
 12. The apparatus of claim 10 wherein the source of concentratedoxygen gas comprises: an air separator for separating concentratedoxygen gas from ambient air input.
 13. The apparatus of claim 10 furthercomprising: a particulate filter in fluid communication with the coolerand the condenser, the particulate filter receiving cooled exhaust gasesfrom the cooler and removing particulate matter from the cooled exhaustgases before the cooled exhaust gases enter the condenser.
 14. Theapparatus of claim 10 further comprising: a blower located in theexhaust gas recirculation conduit for increasing the pressure to induceflow of the first portion of cooled and dried exhaust gases.
 15. Theapparatus of claim 10 further comprising: a gas heat exchanger having afirst section in fluid communication with the combustor and the coolerand a second section in fluid communication with the exhaust gasrecirculation conduit and the combustor, the gas heat exchangertransferring heat from the exhaust gases from the combustor to the gasmixture in the exhaust gas recirculation conduit.
 16. The apparatus ofclaim 15 further comprising: a gas mixer in fluid communication with thecombustor and the first section of the heat exchanger and in fluidcommunication with the exhaust gas recirculation conduit, the gas mixermixing the exhaust gases received from the combustor with an amount ofthe first portion of cooled and dried exhaust gases received from theexhaust gas recirculation conduit.
 17. The apparatus of claim 10 furthercomprising: an exhaust gas recovery conduit in fluid communication withthe condenser gas output conduit for receiving a second portion ofcooled and dried exhaust gases from the condenser gas output conduit;and a carbon dioxide removal system in fluid communication with theexhaust gas recovery conduit, the carbon dioxide removal systemreceiving the second portion of cooled and dried exhaust gases from theexhaust gas recovery conduit and recovering carbon dioxide from thesecond portion of cooled and dried exhaust gases.
 18. The apparatus ofclaim 17 wherein the carbon dioxide removal system comprises: acompressor in fluid communication with the exhaust gas recovery conduitfor compressing the second portion of cooled and dried exhaust gasesreceived from the exhaust gas recovery conduit; and a recovery heatexchanger in fluid communication with the compressor, the recovery heatexchanger receiving the compressed second portion of cooled and driedexhaust gases from the compressor and recovering liquid carbon dioxidefrom the compressed second portion of cooled and dried exhaust gases.19. The apparatus of claim 18 wherein the carbon dioxide removal systemfurther comprises: a gas cleaner in fluid communication with the exhaustgas recovery conduit and the compressor, the gas cleaner receiving thesecond portion of cooled and dried exhaust gases from the exhaust gasrecovery conduit and removing trace gases from the second portion ofcooled and dried exhaust gases before the second portion of cooled anddried exhaust gases enters the compressor.